Asynchronous transfer mode (ATM) is a high bandwidth, low-delay, packet-like switching and multiplexing technique for transferring data across a network using fixed sized cells. The fixed sized cells require lower processing overhead and allow higher transmission speeds than traditional packet switching methods. Each ATM cell contains a payload portion and a header, the latter including a label that associates the cell with a logical communication link between sending and receiving network end systems, i.e., the cell's virtual circuit. Thus, the virtual circuit label for an ATM cell forms the basis on which the ATM cell is routed, or switched, at each network node it encounters.
The ATM cells are asynchronous in that the ATM cells belonging to the same virtual circuit do not necessarily appear in fixed time slots at periodic intervals on the transmission medium. Rather, the position of the ATM cells associated with a particular virtual circuit is random and depends upon the activity of the network. Thus, an ATM system can automatically adjust the network capacity to efficiently transfer data having significantly different data rates, such as voice, computer data, video services, and so forth.
The increasing demand for high and varying data rate communications services, such as TV conference, video information retrieval, or computer data transfer, has created a need for a switching technology that can meet the higher and varying data rates of such broadband services. ATM switching technology is desirable because it offers an acceptable compromise in combining timeliness characteristics (normally associated with circuit switching technologies) and statistical advantages (associated with packet switching technologies). Thus, ATM holds out the prospect of a single transfer mode technology for carrying all traffic types, including voice, entertainment services, or computer traffic.
One component of the ATM network is a core switch system. The core switch system is responsible for switching high traffic volumes, i.e., many data streams of ATM data cells associated with many virtual circuits. This switch typically multiplexes multiple access switches into higher rate paths, such as OC-48 rates (2.5 gigabits per second). The core switch system then is responsible for switching many ATM data streams that may have data rates of 2.5 gigabits per second and higher.
Some conventional ATM switching products include 8.times.8 (eight input by eight output) or 16.times.16 (sixteen input by sixteen output) single stage switch fabrics. These conventional ATM switching products are typically capable of switching input data streams having data rates at either 155 megabits per second or 622 megabits per second. Unfortunately, the conventional single-stage 8.times.8 and 16.times.16 switch fabrics are unable to meet the demands of high data rates and a large number of input-output ports.
In order to increase the number of input-output ports in a switch system, some prior art switch systems incorporate smaller single stage switch fabrics into a multi-stage switch system. Unfortunately, the larger multi-stage switches pose unique problems of interconnectability due to the geometrical increase in hardware cost as the number of input and output ports is increased. The problems associated with interconnection of multiple switch fabrics to form a larger multi-stage switch configuration are: input/output transmission delay variation between the switching fabrics speed degradation, crosstalk, signal path impedance, supply voltage distribution, power dissipation, switch size, and higher cost. These problems are exacerbated by the high data rates of the ATM data streams.
In addition, switching system speed and physical configuration are limited by conventional off-fabric interconnection technology. For example, the conventional ATM switching products use either fiber optical cable, coaxial, or twisted pair cable to interconnect circuit cards within a switch system. This scheme works well for conventional 8.times.8 or 16.times.16 switch fabrics in which only a few data streams are routed to a common switching fabric module. However, this scheme does not accommodate terminating a much larger number of input-output ports due to limitations in connector size, and the size and quantity of interconnecting cables. In other words, a limiting factor in conventional switching systems is the volume of interconnects to a common switching fabric module. Larger switching fabric modules will need to accommodate many more interconnects than can be accommodated in conventional switching systems.
In order to effectively switch data streams, switching systems must operate synchronously. In other words, individual cells of the ATM data streams must be clocked into the switch fabric at the same time. Differing propagation delays due to differing cable lengths between circuit cards and traces on circuit cards prevent this synchronous clocking and switching of the cells.
This problem is circumvented in conventional switch fabrics by making the interconnecting cables the same length and by making traces on the circuit cards of equal lengths. Thus, the propagation delay is the same for all of the ATM input data streams and synchronous arrival of data cells is achieved. However, this scheme also does not economically accommodate the transmission of a much larger number of input-output data streams. The increased quantity of cabling and the circuitous trace layout on the circuit cards significantly increase switch system size and complexity and adversely affect system performance and cost.
Future broadband video services will supply thousands of subscribers with a wide selection of channels. The number of required channels is especially high with video-on-demand and point-to-point video services. These broadband services will require large switches with hundreds or thousands of input-output ports. In addition, as with any service, customer satisfaction is a prime concern thus high quality call connections with little cell loss and high probability for call completion are of significant concern. So, these large switches will desirably provide broadcasting capability while exhibiting very low blocking.
Thus, what is needed is a single stage switch system capable of switching a large number of high speed input data streams. Furthermore, what is needed is a switch system that is non-blocking. In addition, what is needed is an apparatus that is readily miniaturized, highly reliable, and low cost.